1) Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine, which controls the air/fuel ratio of the internal combustion engine by using, as feedback signals, detection signals from an oxygen density sensor (hereinafter called "O.sub.2 sensor") arranged in the exhaust system of the internal combustion engine which may hereinafter be called "engine"as needed. This invention also relates to an improvement in an O.sub.2 sensor employed in such an air/fuel ratio control.
2) Description of the Related Art
A variety of air/fuel ratio control systems making use of O.sub.2 sensors have heretofore been proposed for internal combustion engines. In air/fuel ratio control systems of the above sort for internal combustion engines, an O.sub.2 sensor which has been designed to change its output value abruptly near the stoichiometric fuel ratio by using the principle of oxygen concentration cells of a solid electrolyte, is arranged in an engine exhaust system at an upstream side relative to the point of arrangement of a catalytic converter (three-way catalyst) in the engine exhaust system. The air/fuel ratio of the internal combustion engine is controlled by comparing an output from the O.sub.2 sensor with a predetermined standard value (Which is given as an intermediate value of values between which the abrupt change takes place, and this value is useful as a value for the judgement of either a rich air-fuel mixture or a lean air-fuel mixture) and then controlling the quantity of the fuel to be injected from each electromagnetic fuel injection valve (injector) in such a way that the air-fuel mixture is rendered lean when the output of the O.sub.2 sensor is greater than the standard value but is rendered rich when the output of the O.sub.2 sensor becomes smaller on the contrary.
It has recently been proposed to provide an additional O.sub.2 sensor on the downstream side of the catalytic converter provided in the engine exhaust system (This O.sub.2 sensor will hereinafter be called "rearward O.sub.2 sensor" while an O.sub.2 sensor provided on the upstream side of the catalytic converter like the above-described O.sub.2 sensor will be called a forward O.sub.2 sensor as opposed to the rearward O.sub.2 sensor") and to use an output from the rearward O.sub.2 sensor as auxiliary information for the control of the air/fuel ratio (so-called dual O.sub.2 sensor system or double O.sub.2 sensor system).
Among such conventional O.sub.2 -sensor dependent air/fuel ratio control systems for internal combustion engines, the former systems perform the feedback control of the air/fuel ratio only by the output of a single O.sub.2 sensor and there is hence a room for improvements to the accuracy of the control, and regarding the O.sub.2 sensor employed therein, there are the following additional problems. Since a measuring electrode (platinum or the like) arranged on the side of exhaust gas has lower catalytic ability, the arrival of a non-equlibrated component (for example, CO or HC when the air/fuel ratio is lean) at the measuring electrode causes a reaction such as CO+O.sup.2- CO.sub.2 +e.sup.-. The conventional O.sub.2 sensors therefore generates an output indicating a rich air/fuel ratio so that the static .lambda. point is usually shifted to the lean side. One of causes for such variations of O.sub.2 sensors is considered to be attributable to variations in the catalytic ability of measuring electrodes, resulting in the proposal of O.sub.2 sensors with improved catalytic ability around their O.sub.2 sensor electrodes in recent years. These O.sub.2 sensors are however accompanied with the potential problems that the responsibility of the sensor output may be lowered and greater variations may take place in the deterioration of their performance.
In the latter systems, namely, the dual O.sub.2 sensor systems using a forward O.sub.2 sensor and a rearward O.sub.2 sensor, the temperature on a downstream side of the catalytic converter tends to become lower without exception so that the output of the rearward O.sub.2 sensor does not become stable. Even when the output from the rearward O.sub.2 sensor is used as auxiliary information for the control of the air/fuel ratio, the air/fuel ratio feedback control based on the output of the forward O.sub.2 sensor may not be successfully performed in some instances. Here again, there is a room for improvements. Further, the catalytic converter has a large O.sub.2 storage capacity. This O.sub.2 storage capacity however changes along with durability deterioration of its catalyst. As a result, the responsibility of the air/fuel ratio control system also varies along with deterioration of the catalyst. A complex correction means therefore becomes indispensable if one wants to always maintain at an optimum value the control factor for the air/fuel ratio control.